As an example of a ball end mill in which the cutting edges are formed at unequal intervals in the circumferential direction, a ball end mill in which, of a plurality of peripheral cutting edges which are formed on the outer circumference of the end mill body, at least one peripheral cutting edge has a helix angle which is different from that of the other peripheral cutting edges, and in which an end cutting edge having, as its rotational trajectory, a hemisphere shape, and having a circular arc-shaped, is formed at the tip of this peripheral cutting edge has been proposed in Patent document 1. In addition, a ball end mill in which, when the point of the ball end mill is viewed from the axial direction, the plurality of end cutting edges are seen to form protruding curves that each have a mutually different radius of curvature and that are formed at unequal intervals in the circumferential direction has been proposed in Patent document 2.
In a ball end mill in which the intervals in a circumferential direction between peripheral cutting edges and end cutting edges are formed unequally in this manner, the periods at which these cutting edges bite into a work piece are non-constant, and the vibrations created by such biting actions cancel each other out. As a result, it is possible to prevent resonance that harms the precision of the finished surface (i.e., what is known as ‘chattering vibration’) from occurring.